Mercury diffusion pump



Dec. 24, 1968 w. s. KREISMAN MERCURY DIFFUSION PUMP Filed Jan. 17. 1967FIG. 2

INVENTOR. WALLACE S. KREISMAN BYK B M ATTORNEYS United States Patent3,417,913 NHZRCURY DIFFUSION PUMP Wallace S. Kreisman, Malden, Mass.,assignor to GCA Corporation, Bedford, Mass., a corporation of DelawareFiled Jan. 17, 1967, Ser. No. 609,804 9 Claims. (Cl. 230-101) ABSTRACTOF THE DISCLOSURE An all-metal mercury diffusion pump capable ofwithstanding high baking temperatures and including an annular waterjacket joined to the barrel of the pump by an expansion joint. The waterjacket also substantially encloses the vacuum fore-line and thecondensate return line, and an isolation chamber separates the pumpboiler from the pump manifold.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

This invention relates in general to vacuum pumps and in particular toan all-metal mercury diffusion pump.

Various forms of mercury diffusion pumps are commercially available andthey fall generally into two categories. One category, generally limitedin its use to laboratory environments consists of glass pumps which areinherently fragile and difiicult or impossible to construct in largesizes. The other category consists of metal pumps which are not sostringently limited in size but are generally so constructed that theycannot be baked at high temperatures. Some attempts have been made toprovide metal pumps which can withstand at least some baking, butbecause integral cold traps are incorporated, baking must be restrictedto relatively low temperatures. Another disadvantage of conventionalmetal pumps is the absence of any means for viewing the pumping fluid orthe condensate.

Insofar as glass pumps are concerned, high temperature baking is clearlyimpractical because differential expansion effects between the inner andouter surfaces lead to breakage during heating and cooling cycles.

It is, therefore, a principal object of the present invention to extendthe low pressure capability of mercury diffusion pumps.

It is another object of the present invention to make feasible theoperation of mercury diffusion pumps at temperatures in excess of thosepresently utilized.

Still another object of the present invention is to simplify thestructure of mercury diffusion pumps and to facilitate their disassemblyfor cleaning and inspection.

A further object of the present invention is to permit the viewing ofpumping fluid and condensate in mercury diffusion pumps.

The present invention consists generally in a pump composed almostentirely of metal, principally relatively low-carbon stainless steel.Although incidental elements of the apparatus are composed of Kovar orPyrex, for all practical purposes, the pump is defined as an all-metalpump. The pump can withstand heating to temperatures in excess of 475 C.resulting in a thorough cleaning "ice of the pump during bakeout undervacuum. The geometry of the jet assembly and the incorporation of ahighly efficient annular water jacket of radically different designgreatly extend the low pressure capability of the pump. Moreover, aglass segment is provided in the return line to permit monitoring of thecondensate return action and a glass port in the boiler enclosure of thepump permits viewing of the surface of the pumping fluid. Specificdesign features such as the incorporation of an isolation chamberbetween the manifold and the boiler and the interposition of anexpansion diaphragm between the pump barrel and the annular Water jacketimplement the efficient operation of the pump.

Perhaps the key component of the pump is the manifold, to the top ofwhich the water jacket and jet assembly are welded in such a fashionthat the manifold is well-cooled during normal pump operation. Theannular base of the water jacket substantially surrounds the manifold,being interrupted only by a plug in the water jacket through which apassage is formed. Portions of the condensate return line and thefore-vacuum line are combined in the walled passage through the waterjacket whereby the dual-purpose line is cooled. The fore-vacuum line iscontinued through an annular passage external to the water jacket whichlengthens the cooling path. For a better understanding of the presentinvention together with other and further objects, features andadvantages, reference should be made to the following specificationwhich should be read in conjunction with the appended drawing in which:

FIGURE 1 is an elevation, partly in section, of a preferred embodimentof the vacuum pump; and

FIGURE 2 is a sectional view taken along the line 22 of FIGURE 1.

In FIG. 1 there is shown an ultra-high vacuum connecting flange 12 bywhich the pump may be joined to the system to be evacuated. The flangemay be of the type known as a Conflat flange and it is Welded to a pumpbarrel 14. The barrel is preferably made of 304 low-carbon stainlesssteel as are all other elements of the pump with the exception of thosespecifically noted as being of other materials. Also, all welded jointsbetween mating elements are made by Heliarc and, wherever practical,within an inert atmosphere Welding chamber filled with a gas such asargon. These precautions are taken to avoid any oxidation.

The pump barrel 14 is enclosed along much of its length by a waterjacket 16. At the upper end of the water jacket 16 a flared portion 18is connected to the pump barrel 14 by means of an expansion diaphragm20. The water jacket 16 is terminated at its lower end adjacent aportion of a pump manifold 22. The outer part of the manifold 22 is asleeve member 23 of larger diameter than that of the water jacket 16.

Welded to the top surface of the manifold is a lower jet assembly whichincludes a lower vapor tube 25 flared back upon itself at a point abovethe manifold. A top jet 26 of relatively small diameter, shaped andcapped to direct vapors in a generally downward direction, an uppervapor tube 27 and a lower jet cap 28 are welded together in a separateassembly designed to fit slidably into the lower vapor tube 25. Abayonet-type joint may be provided to assure proper positioning of theunit. The top jet 26 is disposed at a point within the pump barrel 14 nbelow the upper limit of the water jacket. The lower jet 3 cap 28 isspaced along the upper vapor tube 27 at a considerable distance from thetop jet 26 and it, too, is de signed to direct the vapor streamgenerally downwardly.

Welded to the bottom surface of the manifold is an isolation chamber 30through which the lower vapor tube 25 passes centrally. A pump boiler 32is welded to the isolation chamber 30 in such a manner that a re-entrantconfiguration is provided. Although no detail is shown because thestructures are conventional, the heater for the boiler may be a simpleNichrome wire contained Within a housing 34. Also, the exterior of theboiler may be suitably insulated as by the use of stacked rings ofMarinite- 36 which may be joined together by steel rods. The top jet 26,the upper vapor tube 27, and the cap 28 of the lower jet being weldedtogether may be removed through the top of the pump barrel forinspection or cleaning.

A solid portion or plugged area is formed at a point between the pumpbarrel 14 and the wall 16 of the water jacket. A diagonal opening passesthrough the solid portion to form a walled passage 36 running downwardlyfrom the interior of the pump barrel above the manifold to the enlargedouter annular volume of the manifold.

The walled passage 36 has a dual function. Because it passes through thewater jacket it is well cooled and mercury condensate flows freelythrough the passage to a trap 38 from whence it is returned to theboiler 32 as it rises above the trap level. The trap, of course,prevents vapor from the boiler reaching the pump barrel by a route whichbypasses the central vapor tube 25. To permit viewing of the returningcondensate, a glass segment 40 is incorporated in the mercury returnline 42.

The other function of the walled passage 36 is that of the vacuumfore-line connection. The pumping path is through the passage 36, aroundthe outer annular volume of the manifold to a tube 43 which penetratesthe outer annular volume. The tube 43 communicates with a flangedconnector 45 to which a fore-pump (not shown) is coupled.

Some details of the structure, particularly the vacuum fore-lineconnection, mercury drain lines and water-cooling lines may be seen inFIG. 2. For example, a supply line 44 and a drain line 46 for theannular water jacket are shown as is the flanged fore-line connection 45and the annular pumping path. Any one of several conventional mechanicalpumps may be attached to the fore-line connection 45 to provide a vacuumof approximately 10 to 20 microns.

As has been noted, the preferred pumping fluid is mercury and,typically, a quantity of about 6 lbs. of mercury has proven adequate inthe pump boiler. A glass viewing port 48 which may be sealed to a Kovarmember welded into the wall of the boiler 32 facilitates monitoring ofthe level of mercury in the boiler.

Construction details on the embodiment of the invention shown anddescribed include a height of about 24 inches, a throat of about 3 /2inches and pumping speed of about 230 liters per second. The top jet isrelatively small compared to the lower jet which is spaced at somedistance from the lower jet. The emission of vapor from both jets is ina generally downward direction to enhance the probability of entraininggas molecules. Back diffusion of higher pressure gas is inhibited by thecloser spacing of the lower jet to the barrel and the higher temperatureof vapor emitted from that jet.

The annular water jacket maintains a uniform low temperature to improvethe functioning of the condensation barrel wall 14 as compared toconventionally spiral- Wound cooling coils.

The isolation chamber 30 serves to insulate the hot Joiler 32 from thecold manifold during normal pumping perations. On the other hand, theisolation chamber 30 minimizes heat flow from the hot manifold to theboiler luring bake-out. Bake-out at temperatures in excess of i C. ispractical, and the entire pump from the top lange down to the manifoldmay be baked without difli- 4 culty. Pressures as low as 10- torr. havebeen achieved.

Although what has been described and shown constitutes a preferredembodiment of the present invention, a reading of the foregoingspecification will suggest numerous modifications and alternatives tothose skilled in the art. Such variations are believed to be within thepurview of the present invention which should be limited only by thespirit and scope of the appended claims:

What is claimed is:

1. In a pumping system which includes a volume to be evacuated and afore pump, the combination therewith of a mercury diffusion pump whichcomprises a pump barrel, a jet assembly disposed in said barrel, a Waterjacket enclosing said barrel along at least that portion of its lengthwithin which said jet assembly is disposed, a pump manifold including anannular volume disposed at the base of said barrel and having at least aportion thereof in contact with said water jacket, means for vaporizingsaid mercury, said vapor being passed through said jet assembly andcondensed upon the wall of said barrel, and means comprising in part awalled passage formed through said water jacket and communicatingbetween the interior of said annular volume and said barrel'forconducting condensed mercury back to said vaporizing means.

2. In a pumping system as defined in claim 1 the further combination ofa fore-line connection disposed in said annular volume in communicationby way of said walled passage with said interior of said barrel.

3. In a pumping system as defined in claim 1, the combination in whichsaid jet assembly includes a top jet spaced along said barrel from alower jet, said lower jet being spaced more closely to the interior wallof said pump barrel than said top jet.

4. In a pumping system as defined in claim 3, the combination of capsdisposed on said top jet and said lower jet to direct emitted vapors ina generally downward direction.

5. In a pumping system as defined in claim 4, the combination in whichsaid top jet and the cap on said lower jet may be detached as asub-assembly from said lower jet.

6. In a pumping system as defined in claim 1, the combination in whichsaid means for vaporizing said mercury comprises a boiler and in whichcomprises an isolation chamber is interposed between said manifold andsaid boiler.

7. A mercury diffusion pump for use with a fore-pump comprising a pumpbarrel, an annular water jacket surrounding said pump barrel, anexpansion joint connecting the top of said pump barrel to the top ofsaid water jacket, a pump manifold disposed at the base of said pumpbarrel and also contacted by said water jacket, a jet assembly welded tothe top of said manifold and extending upwardly into said pump barrel,an isolation chamber welded to the bottom of said manifold, a pumpboiler welded to said isolation chamber, said manifold having a centralopening formed therein, permitting communication between the area abovesaid manifold and the area below said manifold, a sleeve membersurrounding said water jacket and forming an outer annular manifoldvolume, a walled passage extending through said water jacket from apoint above said manifold downwardly to said outer annular manifoldvolume, a mercury return line connected from said annular volume to saidboiler whereby mercury may be returned to said boiler and means forconnecting said annular volume to said fore-pump by way of an openingformed in said annular volume.

8. A mercury diffusion pump as defined in claim 7 including atransparent member incorporated in said return line to permit viewing ofmercury return action.

9. A mercury diffusion pump as defined in claim 7 including atransparent member incorporated in the wall of said boiler to permitviewing of the mercury level in said boiler.

5 6 References Cited 3,134,534 5/1964 Jancke et a1. 230101 3,141,6067/1964 Landfors 230101 UNITED STATES PATENTS 1g; 3 gpg et DONLEY I.STOCKING, Primary Examiner.

10 an 7/1951 Alexander 5 WARREN J. KRAUSS, Asszstzmt Examiner. 5/1955Lawrance et a1 230-101 6/1957 Gerow 230-401 X 12/1957 Hiesinger et a1230-101 3/1961 Niiller et a1. 230-101 10

